We have studied the regulation of muscarinic receptor and delta-opioid receptors in two related neurohybrid cell lines, NCB-20 and NG 108-15 cells. Our previous studies have demonstrated that treatments of NCB-20 cells with butyrate and dibutyryl cAMP result in up- and down-regulation in the number of muscarinic acetylcholine receptors (mAChRs), respectively. Our current results show that NCB-20 cells express the mRNA for m1-and m4-mAChRs. Butyrate and dibutyryl cAMP induce a respec- tive increase and decrease in the mRNA levels of m1-and m4-mAChRs. Butyrate does not affect m4-mAChR mRNA in NG 108-15 cells, while dibutyryl cAMP significantly decreases m4-mAChR mRNA in this cell type. We have also studied the molecular mechanisms underlying ethanol-induced up-regulation of mAChRs. Ethanol increases mAChR binding sites and the levels of m4-mAChR mRNA in NG 108-15 cells in a time- and dose-dependent manner. Similar ethanol treatment also up-regulates mAChR binding sites and m4, but not m1-mAChR mRNA in NCB-20 cells. The mAChR up-regulation is closely associated with a decrease in intracellular cAMP content. Additionally, we have further characterized delta-opioid binding sites associated with the nuclei of NG 108-15 cells. Our results suggest that nuclear membrane delta-opioid receptors represent newly synthesized mole- cules enroute to the cell surface, whereas the nuclear matrix contains a subpopulation of internalized delta-sites.